Ok, first off, I've stuck it here because I think it's a computer / IT problem and not entertainment.
I have a

collection of films on DVD, some are burnt as DVD film and some are burnt as data DVD's to get 6 films on one DVD. I gave some to the brother in law to take away to work ( 2 weeks on, 2 weeks off ) he got home to say that his laptop wouldn't play my disks and sometimes not even store bought disks
I have tried the disks in my desktop pc, and sure enough, sometimes the drive / pc doesn't pick up the disk, and sometimes it does, I can't put any specific on the order in which they will or won't play,seems random but I'm betting it's not.
SOOooooooooooo, you clever lot, any ideas, JT, sorry but I don't think it's a lens cleaning issue now that it does the same on my pc
Over to you lot